Characterization of pool boiling mechanisms over micro-patterned surfaces using PIV

The present work addresses hydrodynamic and heat transfer processes of pool boiling over surfaces patterned with micro-cavities, in the context of cooling applications. The cavities are square and have a fixed depth. The variable considered here is the distance between cavities 200 pin < S < 2000 gm. The results show that to assure an effective improvement of the heat transfer, the design of the micro-patterns must balance the positive effect of the micro-patterns in promoting the activation of the nucleation sites with the negative effect of an excessive interaction between them. Particular emphasis is given to the horizontal coalescence of the departed bubbles close to the wall, which is observed to cause the deterioration of the heat transfer coefficient, due to the formation of large vapor bubbles that isolate the surface from the liquid. An innovative analysis of the boiling process from micro-patterned surfaces is proposed here, which combines image post-processing with PIV measurements. Evaluation of the characteristic bubbles velocity obtained by PIV measurements evidences that the optimal pattern which balances the positive effect of increasing the parcel of liquid evaporation and the negative effect of the horizontal coalescence (with a distance between cavities S = 400 gm) also allows a more stable vertical bubble velocity, thus removing the vapor from the surface. However, the velocity of the bubbles for these surfaces is quite low, when compared to that of surfaces with a smaller number of cavities, i.e. with larger S. Hence, the convection induced by bubble motion is reduced in these optimal patterned surfaces, although the heat transfer coefficient is the highest. In line with this, one may argue that the positive effect of increasing nucleation sites (under controlled coalescence) considerably improves the latent heat parcel, which is dominant. This may be so, since the surface pattern promoting the largest velocities (C7, S = 1200 mu m) has the lowest heat transfer coefficient, given the lowest number of active nucleation sites. Additionally, destabilization of the bubbles velocity is induced in this surface, which actually contributes to the deterioration of the heat transfer coefficient. (C) 2013 Elsevier Ltd. All rights reserved.